This invention relates to processes and systems for producing elemental sulfur. In particular, it relates to recovering elemental sulfur from acid gas using an improved modified Claus process.
The Claus process is widely used by the industry for the production of elemental sulfur. The process is designed to carry out the Claus reaction: ##STR1## The reaction is favored by decreased temperature and by removal of elemental sulfur vapor.
In the conventional Claus process, the operating conditions of the reactors in which the Claus reaction is carried out are selected to maintain elemental sulfur in the vapor state. Otherwise, the elemental sulfur would deposit on the catalyst and deactivate it. To assure high conversion levels, the reaction is carried out in a plurality of consecutive reactors. Elemental sulfur is condensed and removed from the effluent of a preceding reactor before it is passed to a subsequent reactor. The removal of sulfur permits the reactors to be maintained at progressively reduced temperatures.
Generally, the desired recovery levels necessitate the use of a modified Claus process which includes a thermal reactor, two catalytic reactors and two low temperature catalytic reactors, such as cold bed adsorption (CBA) reactors. The reaction in a CBA reactor is generally carried out at an inlet temperature range from about 250.degree.-280.degree. F. (121.degree.-138.degree. C.) which results in the condensation of elemental sulfur on the alumina catalyst. The low temperatures in the CBA reactor favor the reaction and the condensation of sulfur removes it from the reaction phase, thereby allowing more H.sub.2 S and SO.sub.2 to react. The sulfur condensing on the catalyst deactivates it. Accordingly, a second CBA reactor is provided so that while the first CBA is in the recovery mode, the second reactor is being regenerated and vice versa.
Although presently acceptable recovery rates can be achieved by the above-described modified Claus process, environmental considerations may soon require improved recovery rates. However, the capital costs and expenditures for the standard modified Claus process are already quite high. In order to keep pace with growing environmental concerns, there exists a need to improve this standard modified Claus process so that higher recovery levels can be obtained without added expenditures.
Recovery levels in standard modified Claus processes are limited by a variety of factors. One such factor is that the recovery is usually significantly diminished by leaks in switching valves. There are two reasons for this. First, effluent from the first catalytic reactor is used as regeneration gas. This regeneration gas is relatively rich in sulfur species: the total sulfur in this stream may be equivalent to about 5 to 10% by volume. Therefore, even a small leak of this stream into the tail gas can significantly increase the sulfur loss. Second, since the regeneration gas is rich in sulfur, the piping for carrying it must be made of expensive noncorrosive materials. This adds to the overall cost of the system. Third, regeneration gas flow is routed to the reactor on regeneration by introducing a pressure differential in the flow stream between the first reactor effluent and the second condenser inlet. This results in a significant pressure difference between the reactor on regeneration and that on absorption. This pressure differential may be as much as 5 psi. It is difficult to find large diameter valves that can give leak-free operation under these conditions. Butterfly valves are not generally satisfactory, and it has been necessary to use very expensive steamjacketed globe valves of German manufacture on the critical valve locations in CBA plants in order to maintain high recovery levels.
Another factor that limits recovery is catalyst deterioration caused by thermal stresses introduced during heating or cooling. Also, unregenerated cold spots in the bed further reduce efficiency of the system. Still further, catalyst efficiency is limited by the equilibrium level of absorbed sulfur upon regeneration.
Thus, the standard modified Claus processes suffer from several drawbacks. There presently exists an unsatisfied need for an improved modified Claus process and system that will increase recovery levels of sulfur without increasing cost and overcome the above-mentioned drawbacks. The present invention achieves these goals, and, in fact, can potentially decrease costs and increase the recovery levels of sulfur.